RECP LABORATORY PERFORMANCE TESTING OVERVIEW

Transcription

1 RECP LABORATORY PERFORMANCE TESTING OVERVIEW C. Joel Sprague, Sr. Engineer TRI/Environmental, Inc., PO Box 9192, Greenville, SC Phone: 864/ Fax: 864/ Sam Allen, Vice President and Jarrett Nelson, Sr. Lab Tech TRI/Environmental, Inc., 9063 Bee Caves Rd, Austin, TX /880-TEST Fax: 512/ ABSTRACT Rolled erosion control products (RECPs) have been used in numerous civil and environmental engineering applications worldwide for more than 30 years. Until recently, index tests and large-scale performance tests have been used for characterizing RECPs. Generally, performance properties have been obtained from infrequent, non-standard largescale tests, and, therefore, must be used cautiously. Variations in the mass per unit area, raw materials, manufacturing processes, and innumerable other product/production components make frequent testing imperative for accurate characterization of RECPs. To this end, a new class of test the bench-scale performance test has been formulated which focuses on testing the RECP/soil system under carefully controlled standard conditions, but on a scale that facilitates lower cost and quicker testing. This paper reviews both existing large-scale performance tests as well as a series of new bench-scale laboratory tests for 1) channel erosion, 2) slope erosion, 3) germination & vegetation enhancement, and 4) biodegradation. INTRODUCTION TO RECPS While conventional erosion control materials ranging from loose straw to rock riprap continue to be used extensively, there are many different prefabricated erosion control systems as well. These include the following: 1.) Light-weight synthetic nets or woven organic meshes; 2.) Organic (straw, coconut, excelsior) fiber mats attached to organic or synthetic netting; 3.) Synthetic fiber mats attached to or sandwiched between synthetic nets; 4.) 2- and 3-dimensional welded or woven synthetic filament mat structures; 5.) 2-layer fabric forms for the "casting" of concrete revetments in-place; 6.) Precast, interlocking and often cabled blocks to form articulating mats; 7.) Composite systems that combine one of these with a geosynthetic filter or reinforcement. Systems 1, 2, 3, 4, and in some cases 7 are classified as RECPs.

2 RECP Advantages. Conventional erosion control systems such as blown mulches often fail to perform satisfactorily, and riprap is extremely costly (and sometimes fails, as well). Therefore, designers are considering other materials, such as RECPs, for challenging erosion control applications and for improved economics. RECPs, used in lieu of or in combination with conventional materials, offer the potential to limit erosion while providing the following advantages over traditional materials: Material Quality Control - RECPs undergo manufacturing quality control in a controlled environment to minimize material variation. Construction Quality Control - Large RECPs rolls can be easily and efficiently deployed. Cost Savings - RECPs are generally less costly to purchase, transport and install than alternative Hard systems. Technical Superiority - RECPs are engineered for optimal performance. Construction Timing - RECPs can be installed quickly. Material Availability - RECPs are easily shipped, competitively priced and readily available to any location. Types of RECPs There are two categories of RECPs - temporary degradable and long-term nondegradable. Temporary, Degradable RECPs. Temporary, degradable materials are made from natural fibers derived from the cultivation of various types of straw/hay or jute, or by the processing of coconut hulls (coir) or wood shavings (excelsior) and are used according to Northcutt (1993) to prevent loss of soil from the seedbed and to enhance the establishment of vegetation where the vegetation alone should provide sufficient site protection once established. Erosion control netting (ECN), open weave meshes (ECM), and erosion control blankets (ECB) are the most common temporary, degradable RECP systems. Non-RECP temporary, degradable systems include dry-blown mulch and hydraulically-applied mulches, as well as fiber roving systems (FRS). Long-term, Nondegradable RECPs. Long-term, nondegradable RECPs furnish erosion protection and extend the erosion control limits of vegetation, soil, rock, or other materials. These materials are manufactured of durable plastic materials. RECP systems may combine multiple, and, perhaps, differently manufactured layers. Common long-term nondegradable RECPs are classified as turf reinforcement mats (TRM). Non-RECP long-term, nondegradable systems include fabric formed revetments (FFR), geocellular confinement systems (GCS), gabions (G), and articulating concrete blocks (ACB). Costs. The approximate relative installed costs of various erosion control systems is presented in Table 1. It is important to recognize that costs vary significantly based on project size, location, application, local labor rates and installation conditions.

3 TABLE 1. Erosion Control Systems Costs (1997 Cost Basis) System* Approx. Cost per Square Yard Installed*** Dry-Blown and Hydraulic Mulching $ $1.00 Meshes and Nets $ $1.00 E.C. Blankets $ $1.50 Fiber Roving $ $3.00 Sod $ $4.00 Turf Reinforcement $ $8.00 Hard Systems** $ $60.00 RECPs in bold. * Excludes subgrade preparation, soil ammendments and seeding operations. ** Includes Geocellular Confinement, Fabric Formed Revetments, Riprap, Gabions, Interlocking Block Mats. *** Cost is very sensitive to freight and labor rates. Testing and Evaluation of RECPs Types of Laboratory Tests and Properties. RECP properties can be evaluated using laboratory or field tests. According to Williams and Luna (1987), tests may be divided into three primary categories: 1. Index tests - index tests are standard tests that may be used to compare the relative material properties of several different RECPs. The RECP properties that are evaluated using index tests are typically not appropriate for design. The primary RECP properties that are measured using index tests are listed in Table 2. Index tests that are not used for quality control are run infrequently and, since only limited data is available, results can only be properly reported and certified as nominal test results - not minimums, MARVS (see next paragraph), or even typicals. 2. Quality Control tests - these are index tests which are performed on a production basis to evaluate product integrity, quality and continuity, and to assess the impact of changes in production methodology on product properties. Typical control tests should include RECP mass per unit area, thickness and tensile properties. Quality control test results can be reported with statistical relevance since they are run with great frequency. Commonly, minimum average roll values (MARVs) reflecting a 95% confidence level are reported and specified for quality control tests. 3. Design/Performance tests - these are tests which are used to evaluate the performance properties of RECPs. These tests typically are performed using boundary conditions that simulate field conditions. These tests are also run infrequently and, therefore, can only report results and provide certifications as nominal test values. Williams and Luna (1987) further observe that where the engineer or designer has a great deal of experience with site soils and construction materials, and where budget constraints

4 prohibit requisite performance testing, index tests may be used in lieu of design tests. The results of the index tests are typically reduced by an appropriate factor of safety that is based upon the designer's experience. Caution should be employed when using the results of index tests for design, as failures are often attributed to improper evaluation of field performance. Standardization of RECP Tests. Until recently, there has been relatively little test standardization related to RECP characterization. In the mid-90s, the Erosion Control Technology Council (ECTC), an organization of RECP manufacturers, initiated a program to identify and establish common terminology and to develop standardized index tests to characterize RECPs as described by Allen (1996). In January 1997, the ECTC completed its efforts and issued a manual of common terminology and recommended standards. Subsequent test development and standardization has been pursued by the ECTC and the American Society for Testing and Materials and has focused on performance-related tests. RECP Performance Criteria. Fifield and Malnor (1990), Fifield (1992), and others have proposed various relationships between various RECP properties and performance. Table 2 presents one such categorization of currently available RECP tests according to specific performance criteria and type of test. TABLE 2. Rolled Erosion Control Product Testing Performance Criteria Soil Protection Retention Vegetation Growth Stability Under Flow Survivability & Durability Type of Test Index Tests Quality Control General Index Mass / Area Stiffness Light Penetration Thickness Tensile Strength Color Porosity Volume / Area Water Absorption Swell Channel Flow Simulation (fixed bed) Compression Resiliency U.V. Resist. Smolder Resistance Design/Performance Rainfall / Runoff Simulation Germination / Vegetation Growth Testing Channel Flow Simulation (erodible bed) Biodegradability Testing Large-Scale Performance Testing of RECPs Slope Erosion & Runoff Reduction. A slope is generally eroded by rainfall impact and sheet

5 runoff forces. Therefore, evaluation of a RECP's ability to protect a soil surface from rainfall is appropriate for slope protection applications. Nonstandard test procedures provide for the measurement of the amount of soil loss caused by rainfall generated by a rainfall simulator. At the same time the increased infiltration or runoff reduction can be measured. Soil type, slope, and rainfall rate and duration can be controlled. In one such procedure, plots measuring 2 ft x 20 ft (0.6 m x 6.0 m) are used to compare the effects of rain on a RECP protected slope versus a control, or unprotected, slope. The slopes can be bare, seeded, or allowed to establish vegetation prior to testing. Wind can also be applied to the slope. Erosion only or erosion plus plant germination and growth can be measured and compared to a control section. Rain is applied to the plots until significant rilling has occurred. Runoff is collected, weighed, dried, and weighed again. The effectiveness of the RECP is based on the dried runoff. Cabalka, et al (1998), Sutherland and Ziegler (1996), Rustom and Weggel (1993a,b), Godfrey and Landphair (1991), and perhaps most importantly ASTM s D describe other laboratory and field systems, respectively, for testing of slope erosion. Recently, a hybrid bench-scale performance test has been developed for the ECTC that allows for quicker and lower cost testing of the RECP/soil system under simulated rainfall conditions. This test will be described in the following section. Shear Strength and Channel Erosion. Shear strength, as used with RECPs, is the resistance to a force applied to the surface of the RECP by flowing water. The higher the shear strength the more stable the RECP will be under more severe flow conditions. Nonstandard test procedures such as those described by Urroz and Israelsen (1994) provide for the measurement of the stress caused by flowing water and visual inspection of the specimen in a laboratory flume. In one such procedure, the mat is fastened to a 10 ft 2 (0.9 m 2 ) test section. Water is then released into the flume at velocities that increase incrementally to about 20 ft/s (6 m/s). Velocity is measured upstream and downstream of the test section and shear is measured directly from the test section using a load cell. Three replications are averaged. Not only does a RECP material need to retain its integrity under the expected flow conditions, it also needs to prevent erosion of underlying soils. Large scale flume testing or field trials have been used to measure this performance property. Nonstandard test procedures provide for the measurement of the amount of soil loss caused by flowing water and visual inspection of the specimen in a relatively flat laboratory flume. In one such procedure, the mat is fastened to an 18 inch (46 cm) bed of compacted soil in a 4 ft. (1.2 m) wide flume. Water is then released into the flume at velocities which increase incrementally to about 20 ft/s (6 m/s), or higher. Average cross-sectional velocities and flow depths are measured at stations along the flume. Shear stress can be calculated from these measurements and related to soil loss. Two replications are commonly done. Israelsen (1994) and Sanders, et.al. (1990) have proposed similar procedures. Northcutt and McFalls (1997), Clopper, et al (1998), and perhaps most importantly ASTM s D describe large-scale field channel erosion testing facilities and associated procedures. Recently, a hybrid bench-scale performance test has been developed by the ECTC that allows for quicker and lower cost testing of the RECP/soil system under simulated channel flow conditions.

6 This test will be described in the following section. Germination/Vegetation Growth and Biodegradability. Because no other generally recognized tests have been available, the ECTC has also recently developed two bench-scale performance tests to characterize germination and vegetative growth and biodegradability associated with RECP/soil systems. These tests will be described in the following section. New Erosion Control Bench-Scale Performance Testing Practices Because of the high cost and extended time periods required for performance testing at largescale research-oriented facilities, currently available performance properties, are generally derived from infrequent, single-specimen, non-standard tests and, therefore, must be used cautiously. Variations in the mass per unit area, raw materials, manufacturing processes, and other product/production components make frequent testing imperative for accurate characterization of RECPs. To this end, a new class of test the bench-scale performance test has been developed which focuses on testing the RECP/soil system under carefully controlled standard conditions, but on a scale that facilitates lower cost and quicker testing. These bench, or small-scale, laboratory tests were initiated by the ECTC for 1) slope erosion, 2) channel erosion, 3) germination & vegetation enhancement, and 4) biodegradation. ECTC s New Bench-Scale Slope Erosion Test. This new test method titled, Determination of RECP Performance in Protecting Soil from Rainsplash establishes the guidelines, requirements and procedures for evaluating the ability of RECPs to protect soils from rainsplash-induced erosion. The critical element of this protection is the ability of the RECP to absorb the impact force of raindrops and localized sheet flow, thereby reducing soil particle loosening through splash mechanisms. The test method utilizes bench-scale testing procedures, rather than full-scale simulation, and may not be indicative of conditions typically found on construction sites. Yet, the test method does provide a comparative evaluation of RECPs and bare soil performance under controlled and documented conditions. Additionally, the test method can be used to identify the physical attributes of RECPs that contribute to their erosion control success. Containers containing both bare and RECP-protected soil are exposed to simulated rainfall. Rainfall is simulated using a laboratory drip-type simulator capable of creating uniform drops with a median diameter of 3.0 to 3.5 mm from a drop height of 2.0 m and producing a rainfall intensity as high as 120 ± 11 mm/hr. The amount of soil that splashes out of the containers is collected and weighed. The results can be used to compare bare and RECP-protected situations. The results of the testing include the amount of soil lost from the exposed surface area as shown in Figure 2. From this data an appropriate C-factor can be calculated as the ratio of the RECP-protected soil loss to the control for a given soil, slope and rainfall intensity.

7 FIGURE 1. Bench-scale slope simulation Cumulative Soil Loss (g) Soil Loss via 30 minute Rainfall Simulation 75 mm/hr & 3: Straw w/ 1 75 mm/hr & 3: Time (min) FIGURE 2. Example output - slope protection ECTC s New Bench-Scale Channel Erosion Test. This new test method titled, Determination of RECP Performance under Shear Stress establishes the guidelines, requirements and procedures for evaluating the ability of RECPs to protect soils from flowinduced erosion. As with the bench-scale slope erosion test, this test method utilizes smallscale testing procedures, rather than full-scale simulation, and may not be indicative of conditions typically found on construction sites. Yet, the test method provides a comparative evaluation of RECPs and bare soil conditions under controlled and documented conditions. This test method evaluates an RECP s ability to reduce erosion caused by water flow. This test method can also be used to identify the physical attributes of RECPs that contribute to their erosion control success. Containers containing both bare and RECP-protected soil are immersed in water and subjected to shear stresses caused by the rotation of an impeller with three blades. The shear stress test apparatus includes a tank, test well, motor, transition cover plate, impeller, pressure gage, and pressure regulator as shown in Figure 3. The sample test well is a recession in the floor of the tank that will hold cups prepared for testing. When cups are placed in the well, the test surface is flush with the floor of the tank. The apparatus is

8 typically run at shear stresses well below, near, and well above the shear stress that causes a threshold amount of soil loss. The amount of soil that erodes is found from weighing the containers. The results of the testing include the amount of soil lost from the test pots as shown in Figure 4. From this data an appropriate permissible shear can be determined by comparing the RECP-protected soil loss to a threshold amount of soil loss. Although subjective, the equivalent weight of 13 (½-in) mm of soil loss is used as the threshold. 450 grams is equivalent to 13 mm (½-in) of soil loss. FIGURE 3. Bench-scale channel simulation 675 SOIL ONLY Straw w/ 2 nets g = 13 mm Soil Loss SOIL LOSS (g) SHEAR STRESS (psf) FIGURE 4. Example output - channel protection ECTC s New Bench-Scale Vegetation Enhancement Test. Rolled erosion control products often are used to provide short-term mulching aimed at nurturing vegetation growth. As a result, there is a need to evaluate the effectiveness of candidate RECPs in nurturing initial seed germination. This new test method titled, Determination of RECP Performance in Encouraging Seed Germination and Vegetation Growth establishes the guidelines, requirements and procedures for evaluating the ability of RECPs to encourage seed germination. This test examines the effects of RECPs on seed germination in a simulated, controlled climate. Thus, the results of this test can be used to compare RECPs and other

9 erosion control methods to determine which are the most effective at encouraging the growth of vegetation in different climates. Containers of soil are sown with seeds and then covered with an RECP. Containers are then placed in an environmental chamber to regulate and document light, water, and temperature. The rate of germination is measured periodically throughout the test, and the weight of vegetation is calculated at the conclusion of the test. Each RECP under consideration as well as control containers of uncovered soil undergo testing in the simulated climate as shown in Figure 5. The test is conducted using a specific seed mix. Both cool weather mix - Kentucky 31 tall fescue (PLS = 80% ± 5%) - and warm weather mix - common hulled Bermuda (PLS = 80% ± 5%) - have been used. The ratio, as a percentage, of the weight of vegetation using an RECP to the weight from the control pots is the vegetation enhancement. Figure 5. Bench-scale germination and biodegradation testing ECTC s New Bench-Scale Biodegradability Test. Rolled erosion control products may be used to provide short-term soil protection while nurturing vegetation growth, or they may be required to provide additional long-term turf reinforcement. As a result, there is a need to evaluate the degradability of candidate RECPs in order to classify their suitability for shortor long-term use. This new test method titled, Determination of the Biodegradability of RECPs by CO 2 Release establishes the guidelines, requirements and procedures for evaluating the ability of RECPs to resist biodegradation. In the simplest context the plant materials used in some RECPs are nutrients for microorganisms. Yet the materials are a heterogeneous mixture of substrates, some of which are more readily degraded than others. For example, plants have large portions of cellulose and lignin, the former being much more readily degradable than the latter. Any biodegradation test will measure the sum of the degradation of the multiple substrates, and the more readily degradable materials will disappear first and at a higher rate. An analogy would be the putrefaction of a mammalian carcass - the flesh would go first, leaving the skeleton to persist for a much longer time. The interpretation of biodegradation tests likewise should consider that some substrates in the plant materials will degrade rapidly, and the 9

10 relative extent of biodegradation will depend on the abundance of these readily degradable substrates. However, in all cases there will be more recalcitrant substrates in the plant materials that will persist longer than the investigator is willing to measure. This test method describes the procedures for evaluating the biodegradation of rolled erosion control products (RECPs) by measuring the release of carbon dioxide from the ultimate biodegradation or mineralization of the organic material used in RECPs. The conditions represent those in erosion control applications, but do not duplicate all conditions such as temperatures, moisture levels, soil types (nutrient conditions), sunlight (photodegradation), and the presence of insects. The activity of insects is an important aspect of degradation of these mats. However, a test method to evaluate insect participation would be complex and perhaps specific only to geographical locations. This method monitors only bacterial and fungal activities on the plant materials. Total carbon content of samples of the tested RECP is determined via a gravimetric method. Samples are exposed to inoculum causing biodegradation and associated CO 2 generation to take place. Flasks containing RECP and innoculum are monitored periodically for CO 2 generation and compared to the theoretical total carbon content. A typical series of flasks is shown in Figure 5. Percent degradation plotted on a semi-log plot with a linear trendline has been used to estimate the material's half-life as shown in Figure 6. This half-life is also known as the material's functional longevity. 50% 45% 40% Straw 35% Percent Biodegraded (%) 30% 25% 20% 15% Pure Cellulose 10% 5% 0% Time (days) FIGURE 6. Determination of RECP biodegradation Critique of New ECTC Erosion Control Performance Tests Erosion control designers have been forced to use "ball park" rather than project-specific 10

11 values for "C protected ", Manning's "n", and permissible shear/velocity as primary inputs to the USLE/RUSLE and Manning's equations. Common "ball park" values have historically been derived from a limited number of large-scale laboratory and field tests because projectspecific testing is very costly and time consuming. The new ECTC bench-scale performance tests provide the following advantages over existing performance tests: Multi-specimen bench-scale performance tests facilitate a statistical approach to characterizing product performance. Quicker turnaround times and lower cost are available, thus project-specific testing is now possible Measures soil loss vs. time for multiple events, thus can implement project-specific soil loss / failure criteria Additionally, more thorough product characterization is possible with the biodegradation and vegetation enhancement test methods Because of these advantages, many more variables can be investigated, including: soil type density of soil various kinds of vegetation various degrees of vegetation different slopes & shear stresses, test durations, storm events, etc. The initial results from the bench-scale slope and channel simulation tests appear to be consistent with expectations as detailed in Tables 3, 4 and 5. Industry expectations for vegetation enhancement and longevity are mostly qualitative at this time, but the bench-scale results appear to be consistent with general expectations for biodegradable RECPs. CONCLUSIONS The recent growth and development of the Rolled Erosion Control Products (RECP) industry has created the need for a method to compare the wide array of products now available and to address the fundamental issue of how products perform. Currently available performance properties, are generally derived from infrequent, single-specimen, non-standard tests, and, therefore, must be used cautiously. A new class of test the bench-scale performance test has been formulated which focuses on testing the RECP/soil system under carefully controlled standard conditions, but on a scale that facilitates lower cost, multiple specimens, and quicker testing. These new bench-scale performance tests provide a practical, affordable means to quantify product performance. TABLE 3. Typical C-factors for various slope treatments (After Smith and Ports 1976 & IECA 1996) Dry Mulch Slope C-Factor For Growing Period** 11

12 % < 6 Treatment Rate Annualized* kg/m 2 Wks Mos. Mos. No mulching or seeding all Seeded grasses none all < < < Second Year Grass - all Organic/Synthetic Blankets - all Composite Mats - all Synthetic Mats - all Fully Vegetated Mats - all * annualized C-Factor = (<6 wks value x 6/52) + (1.5-6 mos. value x 20/52) + (6-12 mos value x 26/52); ** approximate time periods for humid climates; Conversion: kg/m 2 x 4.46 = ton/acre TABLE 4. Permissible shear stress for long-term nondegradable RECPs (ECTC 1997) TRM Gray ECTC HEC CIRIA Units Veg 15 1 & Phase Sotir 2 3 TTI 4 GA DOT 5 NAG 6 SI 7 Recommended Design Values Unveg. Unveg. n/a n/a n/a n/a n/a = 8 10 fps Maximum Partial n/a n/a n/a n/a n/a Partly Veg. Velocity n/a Veg. = fps (fps) Fully Fully Veg. n/a n/a n/a Veg. = fps Unveg. Unveg. 2 3 n/a n/a = 2 4 psf Maximum Partial n/a 4 6 n/a n/a n/a Partly Veg. Shear Stress n/a Veg. = 4 6 psf (psf) Fully Fully Veg. n/a 8 10 n/a 8 n/a Veg. = 5 10 psf 1. Chen, Y.H. and Cotton, B.A. (1988), 2. Gray, D.H. and Sotir, R.B. (1996), 3. Hewlett, H.W., et al (1987), 4. Northcutt, P.J. and McFalls, J. (1997), 5. Keller, E.J. and Middlebrooks, P. (1989), 6. North American Green, Inc. (1993), 7. Austin, D.N. and Ward, L.E. (1996), 8. Values interpolated from unvegetated and fully vegetated TRM results. TABLE 5. Permissible shear stress for temporary degradable RECPs 1 (ECTC 1997) Category Product Type Permissible Shear 12

13 Stress Unvegetated Low Velocity Erosion Control Net psf Erosion Control Mesh psf Erosion Control Blanket Single Net psf High Velocity Erosion Control Blanket Double Net Adapted from Gray, D.H. and Sotir, R.B. (1996) & Austin, D.N. and Ward, L.E. (1996) REFERENCES ASTM, 1999, ASTM D : Standard Test Method for Determination of Erosion Control Blanket (ECB) Performance in Protecting Hillslopes from Rainfall-Induced Erosion, American Society for Testing and Materials, West Conshohocken, PA. ASTM, 2000, ASTM D : Standard Test Method for Determination of Erosion Control Blanket (ECB) Performance in Protecting Earthen Channels from Stormwater- Induced Erosion, American Society for Testing and Materials, West Conshohocken, PA. Allen, S.R., 1996, Evaluation and Standardization of Rolled Erosion Control products, Proc. Of GRI-9 Conference on Geosynthetics in Infrastructure Enhancement and Remediation, RM Koerner & GR Koerner, Eds., GII Publ, Phil, PA, pp Austin, D.N. and Ward, L.E., 1996, ECTC Provides Guidelines for Rolled Erosion Control Products, Geotechnical Fabrics Report, Jan.-Feb. Cabalka, D.A., Clopper, P.E., Johnson, A.G., and M.T. Vielleux, 1998, Research, Development, and Implementation of Test Protocols for Rainfall Erosion Facilities (REFS), Proceedings of Conference XXIX, IECA, Reno, pp Chen, Y.H. and G.K. Cotton (1988), "Design of Roadside Channels with Flexible Linings", Hydraulic Engineering Circular No. 15, Report No. FHWA-IP-87-7, Federal Highway Administration, Washington, D.C. Clopper, P.E., Cabalka, D.A., and A.G. Johnson (1998), Research, Development, and Implementation of Performance Testing Protocols for Channel Erosion Research Facilities (CERFS), Proceedings of Conference XXIX, IECA, Reno, pp ECTC, 1997, Letter to the FHWA providing Documentation for HEC #15 Updating, Erosion Control Technology Council, c/o Tim Lancaster, North American Green, Inc. Fifield, J.S. (1992), "How Effective are Erosion Control Products in Assisting with Dry Land Grass Establishment with No Irrigation", Proceedings of Conference XXIII, International Erosion Control Association, Reno, NV, pp

14 Fifield, J.S. and Malnor, L.K. (1990), "Erosion Control Materials vs A Semiarid Environment, What has Been Learned from the Years of Testing", Proceedings of Conference XXI, International Erosion Control Association, Washington, DC, pp Godfrey, S.H. and Landphair, H.C., 1991, "Temporary Erosion Control Materials Testing", Proc. of Conference XXII, International Erosion Control Association, Orlando, pp Gray, D.H. and Sotir, R.B., 1996, Biotechnical and Soil Bioengineering Slope Stabilization: A Practical Guide for Erosion Control, John Wiley & Sons, pp. 30, 35, 329. Hewlett, H.W.M., L.G. Borman and M.E. Brambley, 1987, Design of Reinforced Grass Waterways, Construction Industry Research and Information Association (CIRIA) Report 116, London, England. IECA, 1996, "Practical Approaches for Effective Erosion and Sediment Control," Short Course Notes, IECA, Steamboat Springs, CO. Israelsen, C.E., 1994, "Evaluating the Effectiveness of Erosion Control Products and Practices", Land and Water, Jan/Feb, pp Keller, E.J. and Middlebrooks, P., 1989, Channel Protection Criteria Update, Office of Materials and Research, Georgia Department of Transportation USDOT/FHWA, April. North American Green, Inc., 1993, High Velocity Flow Tests of Two Root-Reinforcing Materials under Bare and Sodded Conditions, Utah Water Research Laboratory, Utah State University, October-November. Northcutt, P.E., 1993, "Field Performance Testing of Roll-Type Erosion Control Blankets Through the Erosion Control Field Laboratory", IECA, Proceedings of Conference XXIV, IECA, Indianapolis, pp Northcutt, P.J. and McFalls, J., 1997, Field Performance Testing of Selected Erosion Control Products Final Performance Analysis Through the 1996 Evaluation Cycle, TXDOT/TTI Hydraulics and Erosion Control Laboratory, February. Rustom, R.N. and Weggel, J.R., 1993a, "A Study of Erosion Control Systems: Experimental Apparatus", Proceedings of Conference XXIV, Indianapolis, pp Rustom, R.N. and Weggel, J.R., 1993b, "A Study of Erosion Control Systems: Experimental Results", Proc. of Conference XXIV, Int'l Erosion Control Assn., Indianapolis, pp Sanders, T.G., Abt, S.R., and Clopper, P.E., 1990, "A Quantitative Test of Erosion Control Materials", Proceedings of Conference XXI, IECA, Washington, D.C., pp

15 Smith, J.O. and Ports, M.A., 1976, "Maryland highway erosion and sediment control: evaluation and future directions," Land Application of Waste Materials, SCSA, Ankeny, IA, pp Sutherland, R.A. and A.D. Ziegler, 1996, Geotextile Effectiveness in Reducing Interrill Runoff and Sediment Flux, Proceedings of Conference XXVII, IECA, Seattle, pp Urroz, G.E. and Israelsen, C.E. (1994), "Direct Measurement of Shear on Erosion Control Mats", Proceedings of Conference XXV, IECA, Reno, NV, pp Williams, N.D. and Luna, J. (1987), "Selection of Geotextiles for Use with Synthetic Drainage Products", Geotextiles and Geomembranes, Vol. 5, pp